Polymethylmethacrylate (PMMA) bone cements are based on the pioneering work of J. Charnley (Charnley, J.: Anchorage of the femoral head prosthesis of the shaft of the femur. J. Bone Joint Surg. 42 (1960) 28-30.). PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator dissolved therein. The powder component, also referred to as bone cement powder, comprises one or more polymers, such as PMMA, produced based on methylmethacrylate, a radiopaquer, and the initiator. Mixing the powder component and the monomer component, swelling of the polymers of the powder component in the methylmethacrylate generates a dough that can be shaped plastically and is the actual bone cement. The radical polymerisation of the methylmethacrylate is initiated by radicals that are formed during the mixing process. Upon advancing polymerisation of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies.
Bone cements are subdivided into high viscosity, medium viscosity, and low viscosity bone cements, inter alia, according to the time until bone cement dough of the PMMA bone cements attains a tack-free condition (Kuhn, K.-D.: Knochenzemente für die Endoprothetik. Springer Verlag, 2001, 18-19.). High viscosity bone cements attain a tack-free condition after 1.0 to 1.5 min., whereas cements attaining a tack-free condition after 1.5 min to 3.0 min are referred to as medium viscosity cements. In low viscosity cements, the cement dough becomes tack-free after more than 3.0 min. The term, “tack-free condition”, is defined in ISO 5833 as the point in time, after the mixing of the cement powder with the monomer liquid, from which the cement dough surface can be touched with a gloved hand without the glove sticking to it.
The bone cement dough can be applied only once it attains a tack-free condition. This means that the point in time at which a tack-free condition is attained indicates the start of the processing phase of the polymethylmethacrylate bone cement. High viscosity and medium viscosity polymethylmethacrylate bone cements are used mainly in the mechanical fixation of large articular endoprostheses, for example of femoral and tibial components of knee-endoprostheses. Low viscosity polymethylmethacrylate bone cements are used with small articular endoprostheses, for example shoulder endoprostheses.
In all previously known cements, the adjustment of the initial viscosity is essentially done by means of specially composed and mutually matched cement powders that have a very well defined content of dibenzoyl peroxide.
The initial viscosity of the polymethylmethacrylate bone cement is reproducible under technical conditions over extended periods of time only with very sophisticated analytical efforts. To date, it is common with all industrially-produced polymethylmethacrylate bone cements that the manufacturer defines the initial viscosity of the polymethylmethacrylate bone cement by means of the composition of the cement powder. Thus far, the medical user has no way of adjusting the viscosity of the polymethylmethacrylate bone cement according to the user's needs during the surgery. In this regard, the specific composition of the known bone cements is limited to a fixed mixing ratio of monomers and powdered PMMA polymer component.
The invention is based on the object to develop a composition of a polymethylmethacrylate-co-polymer bone cement that allows a medical user to adjust the initial viscosity according to his or her needs by means of a single universal cement powder. It was another object to provide a method for application of the universally applicable bone cement as well as a kit that contains said universal cement powder and the monomer component and in which they can be applied directly.
The core of the invention is a specific, universally applicable powdered polymethylmethacrylate-co-polymer, in particular as a component A with a specific content of polymerisation initiator and a radiopaquer as well as, optionally, a pharmaceutically active substance that can be adjusted together with a monomer for radical polymerisation for producing bone cements with an initial viscosity ranging from low viscosity via medium viscosity to high viscosity.